I understand that you wanted to connect Amazon's Dot to Guest WiFi, but you would not be able to connect it that way, however if you want this device connected to Guest WLAN as it is open, then create an hidden SSID secured without passphrase create L2 Access Control with device MAC address, you should be able to connect the Dot to this SSID. ( If required you can set same vlan as guest wifi if you have any policy set on firewall or filter)